Typically, the solid-polymer type fuel-cell stack includes a unit cell in which an electrolyte/electrode structure having electrodes arranged on both sides of an electrolyte is held by metal separators. A laminate having a plurality of unit cells laminated is accommodated in a box-like casing. As this type of fuel-cell stacks, a fuel-cell stack has been invented, which allows, using metal separators, reduction in size and weight and provision of a desired power-generation performance and sealing ability to each unit cell.
With a fuel-cell stack of Japanese Unexamined Patent Application No. 2005-044688, for example, a laminate having a plurality of unit cells laminated is accommodated in a box-like casing. The casing includes first and second end plates, four side plates, four angle members for connecting adjacent ends of the four side plates, and first and second linking pins for linking the first and second end plates and the four side plates.
FIG. 29 is a perspective exploded view of the fuel-cell stack disclosed in the above publication No. 2005-044688. FIG. 29 corresponds to FIG. 1 of Japanese Unexamined Patent Application No. 2005-044688. Referring to FIG. 29, a fuel-cell stack 100 includes a laminate 14 having a plurality of unit cells 12 laminated horizontally (direction of arrow A). A terminal plate 16a, an insulating plate 18, and an end plate 20a are arranged at one end of the laminate 14 in the laminating direction (direction of arrow A) in order toward the outside. A terminal plate 16b, an insulative spacer member 22, and an end plate 20b are arranged at the other end of the laminate 14 in the laminating direction in order toward the outside. The fuel-cell 100 is integrally held by a casing 24 formed rectangularly and including end plates 20aand 20b. 
Referring to FIG. 29, each unit cell 12 includes an electrolyte-film/electrode structure (electrolyte/electrode structure) 30 and first and second thin-plate corrugated metal separators 32 and 34 for holding the electrolyte-film/electrode structure 30. The electrolyte-film/electrode structure 30 includes a solid-polymer electrolyte film 42.
Referring to FIG. 29, the casing 24 includes end plates 20a and 20b, four side plates 60a to 60d arranged at the sides of the laminate 14, angle members (L-angles, for example) 62ato 62d that are connecting members for connecting adjacent ends of the side plates 60a to 60d, and linking pins 64a and 64b of different lengths for linking the end plates 20a and 20b and the side plates 60a to 60d. 
The side plates 60a to 60d each are formed with a plurality of threaded holes 74 at both edges in the width direction. On the other hand, holes 76 are formed in each side of the angle members 62a to 62d to correspond to the threaded holes 74. A screw 78 inserted into each hole 76 is meshed with the threaded hole 74, thereby obtaining fixing of the side plates 60a to 60d through the angle members 62a to 62d. The casing 24 is formed in such a way. The spacer member 22 has a rectangular shape having a predetermined dimension to be positioned at the inner periphery of the casing 24. The thickness of the spacer member 22 is adjusted to absorb variation in length of the laminate 14 in the laminating direction so as to allow application of a desired fastening load to the laminate 14.
With the fuel-cell stack disclosed in the above publication No. 2005-044688, the adjacent ends of the four side plates 60a to 60d are fixed by the angle members 62a to 62d through screwing. Optionally, if a bending flange part is formed at an end of the pair of opposed side plates 60a and 60c, and ends of the pair of side plates 60b and 60d opposed to female threads arranged in the flange part are fixed through screwing, for example, the need for the angle members can be eliminated, obtaining simple structure of the casing. Moreover, if a hinge plate having a thickness greater than that of the four side plates 60a to 60d is spot-welded to both ends of the side plates 60a to 60d, a reduction in size and weight of the casing can be obtained without modifying the conventional structure. Such fuel-cell stack having reduced weight is suitably used, particularly, as a power source for electric vehicles.
Typically, when joining metal plates in the surface direction, spot welding or resistance welding is used frequently. In order to achieve a weight reduction and strength, the casing of the fuel-cell stack is formed of a stainless-steel plate or a high tensile-strength steel plate. However, when spot-welding stainless-steel plates having different thicknesses according to the conventional spot welding method, it is difficult to increase the adherence of a spot-welded part, raising a problem of difficult achievement of sufficient joining strength due to occurrence of cavities or expulsion inside the spot-welded part.
Although it can be considered to replace spot welding with laser welding or electron-beam welding, such welding is typically expensive, becoming a factor that pushes up the manufacturing cost. When spot-welding the stainless-steel plates having different thicknesses, if sufficient joining strength to a tensile-shear force and a repeated shear load can be obtained by contriving the spot-welding method, such a contrivance is preferable for the casing of the fuel-cell stack as described above, providing an advantage of allowing a reduction in junctions of spot welding. It can be said that this is a problem to be solved by the present invention.